What is the Diff file syntax

I am currently playing around with parsing diff files, and have yet to come across a solid documentation on diff files.

I am especially interested in specifications. E.g. I don't really understand the lines that look like this (at the beginning of each changed code block):

@@ -296,7 +296,8 @@

I know they have to do with line numbers, and how much lines have changed, but I wasn't really able to figure out the details so far.

What is the syntax of the output diff files (at least, the main parts)?